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Encyclopedia: Intel 80286, Talk:Intel 80286

Intel 80286 (also called iAPX 286 in the Intel programmer's manual for the 286) is an x86-family 16-bit microprocessor that was introduced by Intel on February 1, 1982. It has 134,000 transistors. Initially released in 6 and 8 MHz editions, it was subsequently scaled up to 12.5 MHz. (AMD and Harris later pushed the architecture to speeds as high as 20 MHz and 25 MHz, respectively.) It was widely used in IBM PC compatible computers during the mid 1980s to early 1990s.

The 80286's performance is more than twice that of its predecessors (the Intel 8086 and Intel 80186) per clock cycle. In fact, the performance increase per clock cycle may be the largest among the generations of x86 processors. Calculation of the more complex addressing modes (such as BX+SI) has less clock penalty because it is performed by a special circuit in the 286; the 8086, its predecessor, has to perform effective address calculation in the general ALU, taking many cycles. Also, complex mathematical operations (such as MUL/DIV) take fewer clock cycles compared to the 8086. The 286 is able to address up to 16 MiB of RAM, in contrast to the 1 MiB the 8086 can work with. While DOS machines were able to utilise this additional RAM capability via extended memory emulation, few 286-based computers ever saw more than a megabyte of RAM.

The 286 was designed to run multitasking applications, including communications (such as automated PBXs), real-time process control, and multi-user systems.

The later E-stepping level of the 80286 was a very clean CPU - free of the several significant errata that caused problems for programmers and operating system writers in the earlier B-step and C-step CPU's (common in the AT and AT clones).

An interesting feature of this processor is that it was the first x86 processor with protected mode. Protected mode enabled up to 16 MiB of memory to be addressed by the on-chip linear memory management unit (MMU) with 1 GiB logical address space. The MMU also provided some degree of prevention from (crashed or ill-behaved) applications writing outside their allocated memory zones. However, the 286 could not revert to the basic 8086-compatible "real mode" without resetting the processor. In theory, real mode applications could be directly executed in 16-bit protected mode if certain rules were followed; however, as many DOS programs broke those rules protected mode was not widely used until the appearance of its successor, the 32-bit Intel 80386, which could go back and forth between modes easily. See Protected mode#Real mode application compatibility for more info.

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